The present invention relates to an apparatus and a method for measuring the magnetic field with a high spatial resolution using a charged particle beam in vacuum.
The magnetic head for accommodating an increased capacity of magnetic recording has been increasingly reduced in size. A technique has been reported for measuring the magnetic field existing in a small area such as generated by a magnetic head using a charged particle beam.
An example of observing the magnetic field generated by a magnetic head using the transmission electron microscope is described in IEEE Transaction on Magnetics, Vol. MAG-12, No. 1, January 1976, pp. 34-39. The method of measuring the magnetic field described in this reference has been executed by the procedure described below. First, an electron beam is transmitted through an amorphous film or a carbon film deposited with silver, and a photographic plate is exposed to a bright field image of this film. The image thus obtained is used as a reference image. Next, a magnetic head is placed under an object lens in the microscope so that the electron beam transmitted through the film passes through an area including the magnetic field generated by the head. The same photographic plate that is exposed to the reference image is also exposed to the bright field image thus obtained. The second bright field image to which the photographic plate is exposed has the film particles appearing partially distorted as compared with the reference image. This is due to the fact that the electron beam that has passed through the magnetic field of the magnetic head is bent in orbit under the influence of Lorentz force. The photographic plate is exposed to double images, one in the absence and the other in the presence of the magnetic field, which contains information that the electron beam is bent by the magnetic field.
The intensity of the magnetic field are detected by irradiating a laser beam having a finite spot diameter on the double-exposed photographic plate and projecting Young's interference fringes on a screen placed in the rear. The number and direction of the fringes thus projected are assumed to represent the intensity and direction of the magnetic field associated with the position irradiated with the laser beam.
Also, IEEE Transaction on Magnetics, Vol. MAG-21, No.5, September 1985, pp.1593-1595 discloses a method of measuring the magnetic field using an apparatus similar to the scanning electron microscope. This method is described below.
First, a focused electron beam is scanned on a flat plane of a magnetic head specimen generating a magnetic field. The electron beam is subjected to Lorentz force and bent in orbit as it passes through the magnetic field. The displacement of the electron beam is measured to provide projection data of a cross section of the magnetic field. This measurement is effected repeatedly by turning the magnetic head. The projection data of the cross section of the magnetic field was processed by utilizing the computer tomography technique thereby to obtain the direction and magnitude, that is, the magnetic field distribution of the magnetic field at each point of space.
Further, IEEE Transaction on Magnetics, Vol. MAG-28, No.2, March 1992, pp.1017-1023 discloses a method in which the electron beam is made stroboscopic with a apparatus configuration similar to the one described above, which beam is generated in synchronism with the magnetic head operated with a high frequency. The displacement due to the magnetic field of the stroboscopic electron beam is measured by a semiconductor position detector, and a dynamic result of observation of the magnetic field distribution of the magnetic head has been reported.
Three conventional techniques for measuring the magnetic field were described above. These methods have their problems respectively.
First, the technique using the transmission electron microscope first described above has two problems as described below. One is that information on the magnetic field cannot be obtained for each point of space. The magnetic field information obtained is an integral value of the magnetic field existing in the region where the electron beam has passed. This is by reason of the fact that all the information on the magnetic field along the route of the electron beam is concentrated on the movement of particles on the photographic plate.
Another problem is that the wide dynamic range of the magnetic field intensity to be measured cannot be compatible with the high spatial resolution. This is for the reason mentioned below.
The spot diameter of the laser beam irradiated on the photographic plate corresponds to the spatial resolution of measurement, and the smaller the spot diameter, the higher the spatial resolution. For Young's interference fringes to be generated, however, the image of particles moved on the photographic plate by the magnetic field must be contained in the spot of the laser beam. The spot diameter of the laser beam, therefore, must be larger than the area covered by the movement of particles. Attempting to measure the line integral value of a large magnetic field deteriorates the spatial resolution. If the dynamic range required of the magnetic field intensity of the magnetic head is to be obtained, for example, the spatial resolution that can be obtained is only about 10 .mu.m.
Also, the two conventional techniques described above using the scanning electron microscope pose two problems described below.
One problem is that the spatial resolution is limited to 0.1 .mu.m though improved over the prior art. This is due to the fact that the spatial resolution is dependent on the diameter of the electron beam passing through the magnetic field and that the beam diameter cannot be reduced further.
Another problem is that the measurable limit in the area closest to the specimen generating the magnetic field is 0.2 .mu.m, and measurement in the nearer area or position is impossible. For the positions closest to the specimen to be measured by the prior art, a focused electron beam is required to be scanned in proximity to the specimen. An attempt to scan the electron beam in excessive proximity to the area closest to a specimen plane, however, causes the electron beam to contact the specimen, thereby making the measurement impossible. This is because of the limit of the positioning accuracy of the electron beam and the fact that the specimen and the electron beam are relatively displaced from each other by drifts.